Allow the customisation of the base namespace of the statsd namespace

[ndobromirov]

As it is all stats are aggregated in mongobetween. namespace. If you have a set of pools, all of the once are shown in the current namespace.

We should be able to add a new parameter to modify the namespace, so we can have more granular things that include host name as well. Maybe expose a parameter or something.

[kounat]

Instead of modifying the namespace (thus changing the metric name), I would prefer to instead leverage tags on the metric to append additional information.

We do have a "backdoor" way to add a tag atm -- we parse the label field in the connection string, and add it as a tag to all metrics. We're open to PRs to make this more flexible if needed